Added LowPassNode - right now, just a direct copy of GainNode - needs new processing code
LowPassNode now actually filtering - customization to come
Added #bypass param to LowPassNode
Add AUDIO_SAMPLE_CLIP to keep filter from unchecked instability
Move AUDIO_SAMPLE_CLIP to ext/seqr/jack.hpp
Added filter equation to comment in node/low_pass.hpp
Starting to generalize LowPassNode's processing - hope to be able to generalize to other causal finite filters
Added customization of the feedforward coefficients
Add some thread safety to CPP2RB parameter assignment with CPP2RB_P_LOCK
Rename LowPassNode to CausalFilterNode

--HG--
rename : ext/seqr/node/low_pass.hpp => ext/seqr/node/causal_filter.hpp